1. Field of the Invention
This invention relates to a mark sheet reading apparatus, and more particularly to a mark sheet reading apparatus for reading marks from a mark sheet on which marking areas, which are binary digitized by marking or painting out them or leaving them blank, are provided in rows and columns of a matrix together with timing marks provided for the individual columns or rows. More specifically, the present invention relates to improvements in means of a mark sheet reading apparatus for correcting coordinates of read data when a mark sheet is read or transported in skew.
2. Description of the Related Art
In conventional mark sheet reading apparatus of the type mentioned, the skew of a mark sheet is usually corrected by physically correcting the position of the mark sheet itself by means of a transporting mechanism section.
However, such physical skew correction means is disadvantageous in that, since the transport mechanism section is complicated in structure, the entire mark sheet reading apparatus is increased in size and in cost.
Meanwhile, Japanese Patent Laid-Open Application No. Helsei 3-15970 discloses a different skew correction means which corrects skew by image processing of data obtained by reading a mark sheet. The processing method is schematically illustrated in FIG. 9. Referring to FIG. 9, where the transporting direction of a mark sheet 50 is represented as the x-axis direction and a direction perpendicular to the x-axis direction is represented as the y-axis direction, an image sensor 51 of the line type on which optic elements are arranged in a row is disposed in parallel to the x-axis. A plurality of marking areas in the form of printed rectangular frames are disposed in rows and columns of a matrix on the mark sheet 50 and selectively marked or painted up into marked marking areas 52 while the other marking areas remain unmarked or blank marking areas 53. Further, a series of timing marks 54 each in the form of a marked or painted up printed rectangular frames are provided along a side edge of the mark sheet 50 in register with and indicative of the individual rows of the marking areas in the matrix. In addition, a pair of left and right skew detection marks 55 and 56 each in the form of a marked or painted up printed rectangular frame are provided in a spaced relationship by a fixed distance W from each other at locations forward of the first row of the matrix of the marking areas on the mark sheet 50. The skew detection marks 55 and 56 are positioned on a straight line parallel to the rows of the marking areas of the matrix.
It is assumed now that the mark sheet 50 is positioned in skew with a certain angle .alpha. with respect to the image sensor 51 in FIG. 9. In this instance, the image sensor 51 first reads binary data represented as marks or blanks in all of the marking areas on the mark sheet 50. Then, the two skew detection marks 55 and 56 are detected from the picture element data obtained from the image sensor 51, and the difference in position between the skew detection marks 55 and 56 in the y-axis direction is discriminated. The difference thus discriminated is converted into a number of rows to obtain a skew row number T, and the trigonometric ratio T/W between the skew row number T and the distance W between the two skew detection marks 55 and 56 is calculated and is determined as the skew inclination .alpha. of the mark sheet 50. Then, upon correction of the coordinates of each of the marked marking areas 52 and the blank marking areas 53, a row preceding the row to which the marked marking area 52 or blank marking area 53 in question belongs is referred to. The reference row here is determined in the following manner.
In particular, where the skew row number is represented by T, when the mark sheet 50 is in skew with a right portion thereof displaced upwardly and picture element data of a certain row are read by the image sensor 51, a row preceding by the distance of T/W.times.n rows to the certain row is referred to for a picture element at the distance n rightwardly from the left end one of the picture elements, that is, for a marked marking area 52 or a blank marking area 53 corresponding to the picture element. On the other hand, when the mark sheet 50 is in skew with a left portion thereof displaced upwardly and picture element data of a certain row are read by the image sensor 51, a row preceding by the distance of T/W.times.n rows to the certain row is referred to for a picture element at the distance n leftwardly from the right end one of the picture elements. Here, where the distance to the reference in terms of a number of rows is represented by s, the conventional skew correction means described above presupposes that the trigonometric ratio s/n between s and n is equal to the trigonometric ratio T/W between T and W calculated from the two skew detection marks 55 and 56 which are provided in a spaced relationship from each other.
However, while the distance W between the two skew detection marks 55 and 56 is determined in advance and fixed, the skew row number T is calculated from a difference in the y-axis direction between the positions of the two skew detection marks 55 and 56 detected from within the read data of the image sensor 51. Accordingly, the trigonometric ratio T/W between the distance W and the skew row number T does not accurately indicate the actual inclination of the mark sheet 50. Further, since the relationship s/n=T/W stands only when the inclination .alpha. of the skew of the mark sheet 50 is comparatively small but does not stand when the inclination .alpha. is comparatively great, where the inclination of the skew of the mark sheet 50 is great, the coordinates of the marked marking areas 52 and the blank marking areas 53 are detected in error, and consequently, the skew of the mark sheet 50 cannot be corrected accurately.